*! version 1.0.1  Fri Apr  8 10:47:15 CDT 1994  (STB20: sg25)
program define xi_eii
	version 3.1
	local orig "`1'"
	local ichar "`2'"

	local lstar = index("`orig'","`ichar'")
	local part1 = substr("`orig'",1,`lstar'-1)
	local part2 = substr("`orig'",`lstar'+1,.)
	if upper(substr("`part2'",1,2))!="I." {
		xi_eic `orig' "`ichar'"
		exit
	}
	else if "`ichar'"!="*" { 
		di in red "I.xxx|I.yyy not allowed"
		exit 198
	}
	xi_ei `part1'
	local res1 "$S_1"
	xi_ei `part2'
	local res2 "$S_1"

	if "`res1'"=="." | "`res2'"=="." { 
		di in gr "`orig'" _col(36) "(requires no interaction terms)"
		exit
	}

	_crcunab `res1'
	local uab1 "$S_1"
	parse "`uab1'", parse(" ")
	local len1 0
	while "`1'"!="" {
		if length("`1'")>`len1' {
			local len1 = length("`1'")
		}
		mac shift
	}
	_crcunab `res2'
	local uab2 "$S_1"
	parse "`uab2'", parse(" ")
	local len2 0
	while "`1'"!="" {
		if length("`1'")>`len2' {
			local len2 = length("`1'")
		}
		mac shift
	}

	local len1 = `len1'-length("`res1'") + 1
	local len2 = `len2'-length("`res2'") + 1
	local len = `len1'+`len2'
	local c1 = substr("`res1'",2,1)
	local c2 = substr("`res2'",2,1)
	if `len'==2 {
		local stub "I`c1'X`c2'_"
	}
	else if `len'==3 { 
		local stub "I`c1'`c2'_"
	}
	else if `len'==4 {
		local stub "I`c1'`c2'"
	}
	else {
		di in red "syserr: length |`len'| not right"
		exit 198
	}

	xi_mkun2 `stub'
	local stub "$S_1"

	parse "`uab2'", parse(" ")
	local i 1
	local a : word `i' of `uab1'
	while "`a'"!="" {
		local num1 = substr("`a'",length("`res1'"),.)
		local lbl1 : variable label `a'
		local j 1
		while "``j''"!="" {
			local num2 = substr("``j''",length("`res2'"),.)
			qui gen byte `stub'`num1'_`num2' = `a'*``j''
			local lbl2 : variable label ``j''
			label var `stub'`num1'_`num2' "`lbl1' & `lbl2'"
			local j=`j'+1
		}
		local i=`i'+1
		local a: word `i' of `uab1'
	}
	global S_1 "`res1' `res2' `stub'*"
	di in gr "`orig'" _col(23) "`stub'#-#" /*
		*/ _col(36) "(coded as above)"
end
exit

		
		

	

	

	



I.myvar[what]	means dummies for myvar, drop dummy for myvar==what
I.myvar*thatvar means interaction of myvar and thatvar 
I.myvar[val]*thatvar[val] means drop corresponding.
	
12345678
I123_#x#
I12##x##


For I.name*I.name
We try:
	12345
	IrXr_	e.g., IrXr_#_#   for two 1-digit numbers
	Irr_    e.g., Irr_#_## or Irr_##_# for 1 and 2 digit numbers
	Irr     e.g., Irr##_##   for two 2-digit numbers

For I.name*name
	Irr_#

I.abc*I.def

IrXr_ we try, then shorten to 
Irr_

I12345_#
I1234_##
I1234###